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Calibration of the ATLAS precision muon chambers and study of the decay _t63 [tau] → _m63_m63_m63 [mymymy] at the large hadron collider [Elektronische Ressource] / Jörg Horst Jochen von Loeben

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¨ ¨TECHNISCHE UNIVERSITAT MUNCHENMax-Planck-Institut fu¨r Physik(Werner-Heisenberg-Institut)Calibration of the ATLAS Precision Muon Chambers andStudy of the Decay τ → μμμ at the Large Hadron ColliderJ¨org Horst Jochen von LoebenVollsta¨ndiger Abdruck der von der Fakult¨at fu¨r Physik der Technischen Universita¨tMu¨nchen zur Erlangung des akademischen Grades einesDoktors der Naturwissenschaften (Dr.rer.nat.)genehmigten Dissertation.Vorsitzender: Univ.-Prof.Dr. A. IbarraPru¨fer der Dissertation:1. Priv.-Doz.Dr. H. Kroha2. Jun.-Prof.Dr. L. FabbiettiDie Dissertation wurde am 22. Juni 2010 bei der Technischen Universita¨t Mu¨ncheneingereicht und durch die Fakult¨at fu¨r Physik am 7. Juli 2010 angenommen.AbstractThe Large Hadron Collider (LHC) is designed to collide protons at centre-of-mass energies of up to 14TeV. One of the two general purpose experi-ments at the LHC is ATLAS, built to probe a broad spectrum of physicsprocesses of the Standard Model of particle physics and beyond. ATLASis equipped with a muon spectrometer comprising three superconductingair-core toroid magnets and 1150 precision drift tube (MDT) chambersmeasuring muon trajectories with better than 50m position resolution.The accuracy of the space-to-drift-time relationships of the MDT cham-bers is one of the main contributions to the momentum resolution.

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Published 01 January 2010
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¨ ¨TECHNISCHE UNIVERSITAT MUNCHEN
Max-Planck-Institut fu¨r Physik
(Werner-Heisenberg-Institut)
Calibration of the ATLAS Precision Muon Chambers and
Study of the Decay τ → μμμ at the Large Hadron Collider
J¨org Horst Jochen von Loeben
Vollsta¨ndiger Abdruck der von der Fakult¨at fu¨r Physik der Technischen Universita¨t
Mu¨nchen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr.rer.nat.)
genehmigten Dissertation.
Vorsitzender: Univ.-Prof.Dr. A. Ibarra
Pru¨fer der Dissertation:
1. Priv.-Doz.Dr. H. Kroha
2. Jun.-Prof.Dr. L. Fabbietti
Die Dissertation wurde am 22. Juni 2010 bei der Technischen Universita¨t Mu¨nchen
eingereicht und durch die Fakult¨at fu¨r Physik am 7. Juli 2010 angenommen.Abstract
The Large Hadron Collider (LHC) is designed to collide protons at centre-
of-mass energies of up to 14TeV. One of the two general purpose experi-
ments at the LHC is ATLAS, built to probe a broad spectrum of physics
processes of the Standard Model of particle physics and beyond. ATLAS
is equipped with a muon spectrometer comprising three superconducting
air-core toroid magnets and 1150 precision drift tube (MDT) chambers
measuring muon trajectories with better than 50m position resolution.
The accuracy of the space-to-drift-time relationships of the MDT cham-
bers is one of the main contributions to the momentum resolution. In this
thesis, an improved method for the calibration of the precision drift tube
chambers in magnetic fields has been developed and tested using curved
muon track segments. An accuracy of the drift distance measurement of
better than 20m is achieved leading to negligible deterioration of the
muon momentum resolution.
Thesecond partof thiswork is dedicated to the studyof thelepton flavour
violating decay τ → . Lepton flavour violation is predicted by al-
12most every extension of the Standard Model. About 10 τ leptons are
33 −2 −1produced per year at an instantaneous luminosity of 10 cm s and a
centre-of-mass energyof14TeV.Simulateddatasampleshavebeenusedto
evaluate thesensitivity oftheATLASexperimentforτ →decayswith
−1an integrated luminosity of 10fb . Taking theoretical and experimental
systematic uncertainties into account an upper limit on the signal branch-
−7ing ratio ofB(τ →)<5.910 at 90% confidence level is achievable.
This result represents the first estimation in ATLAS.
iiiContents
1 Introduction 1
2 Theoretical Background 3
2.1 The Standard Model of Particle Physics . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Quantum Chromodynamics . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 The Electroweak Theory . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3 Physics Beyond the Standard Model . . . . . . . . . . . . . . . . . . 8
2.2 Lepton Flavour Violation in the Standard Model . . . . . . . . . . . . . . . 9
2.3 Lepton Flavour Violation in Theories Beyond the Standard Model . . . . . 10
2.3.1 Models with Tree Level Suppressed Lepton Flavour Violation . . . . 11
2.3.2 Models with Tree Level Lepton Flavour Violation . . . . . . . . . . . 12
3 The LHC and ATLAS 15
3.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 The ATLAS Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 Physics Goals and Detector Performance. . . . . . . . . . . . . . . . 17
3.2.2 The ATLAS Detector . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.3 The Magnet System . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.4 The Inner Tracking Detector . . . . . . . . . . . . . . . . . . . . . . 21
3.2.5 The Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.6 The Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3 Monitored Drift Tube Chambers . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.1 Drift Tube Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3.2 Chamber Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.3 MDT Chamber Electronics . . . . . . . . . . . . . . . . . . . . . . . 32
3.3.4 MDT Chamber Naming Scheme . . . . . . . . . . . . . . . . . . . . 34
3.4 Trigger and Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4 Calibration of the MDT Chambers 37
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 The Drift Time Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3 The MDT Calibration Model . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3.1 Required Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3.2 Muon Calibration Stream . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3.3 Performance Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Calibration of the Space-to-Drift-Time Relationship . . . . . . . . . . . . . 45
vvi Contents
4.4.1 Integration Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.4.2 Principle of Autocalibration . . . . . . . . . . . . . . . . . . . . . . . 46
4.5 Autocalibration with Curved Tracks . . . . . . . . . . . . . . . . . . . . . . 47
4.5.1 Working Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.5.2 Curved Track Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.5.3 Sensitivity of the Residuals to the Drift Radii . . . . . . . . . . . . . 51
4.5.4 The Correction Function . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.5.5 Iteration and Convergence . . . . . . . . . . . . . . . . . . . . . . . . 52
4.5.6 Fixed Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.6 Simulated Performance of the Autocalibration . . . . . . . . . . . . . . . . . 53
4.6.1 The Dataset for the Performance Tests . . . . . . . . . . . . . . . . . 54
4.6.2 Testing Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.6.3 Determination of the Convergence Criterion . . . . . . . . . . . . . . 58
4.6.4 Dependence on the Start Values . . . . . . . . . . . . . . . . . . . . 60
4.6.5 Tuning the Autocalibration . . . . . . . . . . . . . . . . . . . . . . . 64
4.6.6 Standard Autocalibration Procedure . . . . . . . . . . . . . . . . . . 66
4.6.7 Dependence on Statistic . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.6.8 Calibration of the Full Spectrometer . . . . . . . . . . . . . . . . . . 69
4.7 Systematic Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.7.1 Effect of the Drift Time Resolution . . . . . . . . . . . . . . . . . . . 69
4.7.2 Effect of the Accuracy of the t Determination . . . . . . . . . . . . 710
4.7.3 Effect of t -Shifts within an MDT Chamber . . . . . . . . . . . . . . 720
4.7.4 Effect of the Chamber Geometry . . . . . . . . . . . . . . . . . . . . 75
4.8 Influence on Momentum Resolution . . . . . . . . . . . . . . . . . . . . . . . 76
4.8.1 Average Momentum Resolution of the Muon Spectrometer . . . . . 77
4.8.2 η- and φ-Dependence of the Momentum Resolution . . . . . . . . . . 81
4.9 Test of the Autocalibration with Cosmic Ray Muons . . . . . . . . . . . . . 84
4.9.1 Cosmic Muons in ATLAS . . . . . . . . . . . . . . . . . . . . . . . . 85
4.9.2 Datasets from Combined Cosmic Runs . . . . . . . . . . . . . . . . . 87
4.9.3 Autocalibration with Cosmic Muons . . . . . . . . . . . . . . . . . . 88
4.9.4 Autocalibration with Magnetic Field . . . . . . . . . . . . . . . . . . 92
4.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
τ →5 Search for LFV Decays with ATLAS 97
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2 Simulation of the Decay τ → . . . . . . . . . . . . . . . . . . . . . . . . 98
5.3 Production of τ Leptons at the LHC . . . . . . . . . . . . . . . . . . . . . . 99
5.3.1 Production of τ Leptons in Heavy Meson Decays . . . . . . . . . . . 99
5.3.2 Gauge Boson Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.4 Signal Event Topology and Detector Acceptance . . . . . . . . . . . . . . . 100
5.4.1 Kinematic Properties of the Signal Events . . . . . . . . . . . . . . . 101
5.5 Background Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.5.1 Background from Charm Decays . . . . . . . . . . . . . . . . . . . . 105
5.5.2 Background from Beauty Decays . . . . . . . . . . . . . . . . . . . . 106
5.6 Simulation of the Detector Response . . . . . . . . . . . . . . . . . . . . . . 108
5.7 Trigger Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.7.1 Efficiency for τ → of the First-Level Muon Trigger . . . . . . . 108Contents vii
5.7.2 Efficiency of the High-Level Triggers . . . . . . . . . . . . . . . . . . 109
5.8 Reconstruction of Physics Objects and Detector Performance . . . . . . . . 111
5.8.1 Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 111
5.8.2 Reconstruction of the Secondary Vertex . . . . . . . . . . . . . . . . 116
5.8.3 Missing Energy Reconstruction . . . . . . . . . . . . . . . . . . . . . 116
5.8.4 Event Display of aτ → Decay . . . . . . . . . . . . . . . . . . . 117
5.9 Signal Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.9.1 Event Pre-Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.9.2 Signal Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
¯5.10 Estimation of the bb Background Rate. . . . . . . . . . . . . . . . . . . . . . 126
5.11 Error Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.11.1 Expected Statistical Uncertainties . . . . . . . . . . . . . . . . . . . 129
5.11.2 Theoretical Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . 129
5.11.3 Expected Systematic Uncertainties . . . . . . . . . . . . . . . . . . . 129
5.12 Upper Limit Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.12.1 Statistical Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.12.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6 Summary 137
A Momentum Measurement with the ATLAS Muon Spectrometer 139
B MDT Chamber Naming Scheme 143
C Autocalibration with Curved Tracks 145
C.1 Analytic Expression for the Residual . . . . . . . . . . . . . . . . . . . . . . 145
C.2 Sensitivity to the Measured Drift Radii . . . . . . . . . . . . . . . . . . . . 147
D The Lorentz Angle Correction Function 149
Bibliography 161Chapter 1
Introduction
Exciting years are awaiting the research field of particle physics. The Large Hadron
Collider (LHC), the most powerful particle accelerator ever constructed, has started its
1operation at the European Laboratory for Particle Physics (CERN ). It is designed to
accelerate proton beams up to 7TeV, allowing for proton-proton collisions with a centre-
of-mass energy of 14TeV. With these collisions, particle physicists from all around the
world hope to find the answers to the most fundamental questions about the laws of na-
ture such as the experimental prove of the last undetected particle of the Standard Model
of particle physics: the Higgs boson. Thisdiscovery is crucial for the validation of the cur-
rent understanding of matter, as this particle is supposed to give mass to all fundamental
particles. Equally interesting questions concern phenomena beyond the Standard Model.
The hope is to find hints towards an even more fundamental theory of nature, including
for example Supersymmetry which can provide a promising dark matter candidate and
elegantly solves the hierarchy problem.
Colliding highly energetic protons is only one part of the exercise, the other one is the
meticulousreconstructionoftheprocessesoccurringinthecollisions. Forthispurpose,the
2LHC is equipped with four particle detectors. The largest one is ATLAS , a general pur-
pose detector which has been designed to exploit the full range of physics accessible at the
LHC. Among its most distinct features is the muon spectrometer in a toroidal magnetic
field created by superconducting air-core magnets. Since muons are produced in many
interesting physics processes at the LHC, the precision of the momentum measurement of
the muon spectrometer plays an important role.
After an introduction of the main properties of the LHC and ATLAS (Chapter 3) a pre-
cise method for the calibration of muon drift chambers in magnetic fields is presented in
Chapter 4. Especially for muon momenta above about 100GeV/c, the muon chamber
calibration is one of the dominating contributions to the muon spectrometer resolution.
The second part of this work is dedicated to the search for lepton flavour violating decays
τ →. Lepton flavour violation is predicted by almost every extension of the Standard
Model, including supersymmetric and so-called Little Higgs models. The lepton flavour
violating decay rates predicted by these theories are becoming accessible at new colliders
like the LHC. Chapter 2 gives an overview about lepton flavour violation in the Standard
Model and in selected theories beyond the Standard Model. The LHC will produce about
1CERN - Conseil Europeen pour la Recherche Nucleaire.
2ATLAS - A Toroidal LHC ApparatuS.
12 Chapter 1. Introduction
1210 τ leptons during one year of stable data-taking, which motivates the study of the
ATLAS sensitivity to the neutrinoless decay τ → presented in Chapter 5. After dis-
cussing the various sources ofτ leptons in proton-proton collisions, a possible strategy for
this search and the expected upper limit on the branching ratio accessible by ATLAS are
discussed.